Add tasks for all the new languages

Task/Modular-exponentiation/Nim/modular-exponentiation.nim

@@ -0,0 +1,18 @@
+import bigints
+
+proc powmod(b, e, m: BigInt): BigInt =
+  assert e >= 0
+  var e = e
+  var b = b
+  result = initBigInt(1)
+  while e > 0:
+    if e mod 2 == 1:
+      result = (result * b) mod m
+    e = e div 2
+    b = (b.pow 2) mod m
+
+var
+  a = initBigInt("2988348162058574136915891421498819466320163312926952423791023078876139")
+  b = initBigInt("2351399303373464486466122544523690094744975233415544072992656881240319")
+
+echo powmod(a, b, 10.pow 40)